Our business currently subscribes to a DSL internet connection. We're moving away from this and in fact will have two 3Mbps dedicated fiber connections run into our operations center in the coming weeks. The first fiber connection will be ready in 2 weeks, the second in 2 months.

My question is with regards to DNS records, specifically in publishing multiple DNS records that point to the same address. When the first fiber connection becomes available I will want to have our domain (www.example.com) point to both it and the old dsl service. I will be using a multi-wan router to handle both connections. When the second fiber connection is installed I will essentially do the same configuration as before, cutting the dsl conn out of the loop. I'm fairly new to DNS and am concerned as to how I might achieve this.

So, with two connections... if one connection fails then will every other customer's connection fail or will they experience momentary latency and then be redirected to the still functioning connection?
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JohnyDSep 23 '09 at 14:01

If one connection fails, one of N customers' connection will fail too, N being the number of IN A records. This is due to DNS caching on the client side. When a client resolves a domain, if there are multiple records, there are multiple answers (randomly sorted). The first answer is then cached for TTL seconds. If you don't update your configuration when a host fails, at the end of the TTL, another query will occur, with 1/N chance to get the same host. Round robin DNS is not fail-over !
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Julien TartarinSep 23 '09 at 17:24

1

"they will be randomly served by your DNS server" is wrong. It depends on the name server (NSD, for instance, does not do it) and, for BIND, it depends on its configuration. Sometimes, the records are randomly shuffled, not by your name server, but by the recursive cache of your clients.
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bortzmeyerSep 24 '09 at 7:45